This improv game is in the following improv game categories:
This one is in the SingSong
category but it's equally a fun warmup.
Everyone in e circle and get a beat running; use a beatbox, cd or mp3. First player starts rapping a line, any line, on the beat. Without skipping too many beats, next player adds a line that rhymes and so on until a player fails to come up with a line, misses the beat or fails to rhyme, after which everybody goes 'Dang', and the next player starts a next round. When the players not rapping feel the rhyme word of the active player coming, they all join in and yell the word together at the end of every line.
For (not improvised) inspiration check out 'Intergalactic' by the Beasty Boys.